Referring to the drawings, FIG. 2 shows a prior art turbine face seal commonly used in air turbine starters. The face seal includes a rotor 2 mounted to shaft 24 adjacent a bearing 26. The rotor 2 has an axially facing flange 3. The face seal further includes a stator 4 mounted to turbine exhaust housing 27. Disposed between the rotor 2 and the stator 4 is a carbon seal ring 5 that sealingly engages the flange 3.
In typical air turbine starters such the ATS 100 shown in FIG. 1, the rotation of the turbine wheel 22 can generate under certain circumstances a low pressure or "vacuum" on the downstream side of the wheel which is also the airside of the turbine seal. This differential pressure results in large oil leakage rates if the turbine seal has any flaws or coke build up in the contact zone. The coke build-up destroys the flatness of the sealing contact surfaces between the rotor and the seal carbon ring. The coke separates the two surfaces resulting in a clearance therebetween. As a result an air/oil mist is pulled from the bearings through the seal and into the turbine exhaust air. If 100 to 300 ccs of oil is displaced to the turbine exhaust air without detection by the pilot or ground crew, loss of the air turbine starter by turbine bearing failure is possible. This sensitivity of the system design to turbine seal flaws results in the turbine seal being in the top three causes of air turbine starter returns from the field for cause.
Accordingly, there is a need for a turbine seal for use in air turbine starters that prevents oil leakage even in the presence of a coke build up.